The monoclonal antibody (mAb) OKT4 identifies a subset of T cells responsible for the induction of immune responses to soluble antigen, commonly called helper T cells. These cells require determinants present on autologous Ia molecules and determinants present on the antigen for activation of the T cell to occur. This dual specificity suggests that this subset of T cells may bear a receptor for autologous Ia determinants. OKT4 has been demonstrated to block activation and certain functions of this T cell subset, and some investigators have suggested that the ligand bound by OKT4, termed T4 may be the T cell Ia receptor. The principal investigator has generated a T4 bearing human T-T hybrid capable of activation by and specific binding of autologous Ia molecules. This hybrid binds autologous but not allogeneic Ia molecules, and this binding is inhibited with anti-Ia mAb's. Additionally, a mAb termed A-11, specific for the hybrid, has been identified and may bind the hybrid Ia receptor. The experiments proposed in this application are designed to study the relationship of T4, the hybrid Ia receptor, and the A-11 ligand. The aim of the studies is to determine whether T4 is or is not the T cell Ia binding structure.